This invention relates to continuously-variable-ratio transmissions (which will be referred to as CVT's) of the toroidal-race, rolling-traction type. It relates in particular to the variators, that is to say the ratio-varying units, of such transmissions in which rollers of variable orientation transmit traction between coaxial and part-toroidal input and output grooves or races, formed on coaxial and rotatable input and output discs respectively. By simultaneously altering the radius from the common axis of the discs at which the rollers make rolling contact with the two races, the relative speeds of the two discs change, resulting in a change in the transmitted ratio. While the prior art teaches and the invention will be described with relation to toruses of circular cross-section, the invention includes CVT's in which the torus is generated by rotating any closed figure, of generally circular outline, about a generator line.
Patent applications in this art, relating especially to automobile transmissions, have been filed regularly from at least the 1920's onwards. Specification U.S. Pat. No. 1,865,102 is an example of a patent granted on one such application filed in 1929. In that specification, as in many others in this art, there are two input races and two output races and a set of three rollers transmits drive from each input race to its corresponding output race, all rollers in the variator being constrained at all times to contact their respective input race at a common first radius and their respective output race at a common second radius.
It has been common practice in the art to mount each roller so that it spins about an axle mounted in a supporting member usually called a carriage, and to connect the carriages of all the rollers in one set so that those carriages move in unison when required so as to change the transmitted ratio, and between such movements to hold their associated rollers steady so that they all transmit the same ratio in the manner already described.
In the accompanying drawings, FIGS. 1 to 3 all show the same, known type of variator. They are also all simplified and generally diagrammatic, and should be studied together because certain parts shown in one of them are omitted from one or both of the others. FIG. 1 is an axial section through the variator, FIG. 2 shows the roller-supporting mechanism in a section on the line II--II in FIG. 1, and FIG. 3 is a section on the line III--III in FIG. 2. As shown in FIG. 1, an input shaft 1 is rotatable about an axis 2, is driven by a prime mover 3 and carries two input discs 4 and 5 formed with part-toroidal races 6 and 7 respectively. Disc 5 is fixed to shaft 1, while a keyed connection 8 prevents mutual rotation between the shaft and disc 4 but allows limited relative axial movement. Disc 4 acts as a piston within a cylindrical cap 9 which is fixed to shaft 1, and the chamber 10 within the cap is connected to a pressurized fluid source 11. A single output disc 13, formed on its opposite faces with part-toroidal races 14 and 15, is mounted in a bearing 16 with freedom to rotate about input shaft 1 and to make limited relative movement axially. Disc 13 constitutes the-output member of the variator and a gear 17, formed on the rim of the disc, engages with the final drive of the transmission (not shown) by way of a gear 18 rotatable on a support fixed relative to the variator casing 19. Race 14 conforms to the surface of the same torus as race 6, and races 15 and 7 are similarly related. A set of three rollers 20, which are equi-spaced around axis 2 but of which only one is shown, make rolling contact with races 6 and 14 and so transmit drive from input disc 4 to output disc 13. Rollers 20 are mounted in a supporting frame 21. A second and symmetrically-arranged set of rollers 25, mounted on a supporting frame 26, transmit drive from race 7 to race 15 formed on the opposite face of output disc 13. The necessary hydraulic end load to urge the discs and rollers firmly into contact with each other by way of an intervening thin film of fluid, so that they transmit the required driving power to the final drive by way of gear 18 in a manner well known in the art, is generated by the fluid in chamber 10. As already stated, input disc 4 and output disc 13 can make slight axial movements in response to that load.
The two roller supporting frames 21 and 26 are essentially similar, and frame 21 is illustrated best in FIG. 2. It comprises a frame member 30 of generally triangular shape having a central aperture 31 to accommodate the shaft 1. Each roller 20 spins about an axis 33 on an axle 32 mounted in a carriage 34 which encompasses the roller along a line 35 passing through the roller centre 22 but leaves the two roller segments that are furthest from that line unobstructed, so that the roller can contact the races 6, 14 as already described. To change the transmitted ratio, each roller and its associated carriage 34 must be able to pivot about the same line 35 with which the carriage 34 is itself aligned, and one of the means well known in the art for inducing such pivotal movement is to impose "tangential shift"--that is to say a movement generally tangential to the centre circle of the common torus of races 6 and 14--upon the roller and carriage. In FIGS. 2 and 3, which illustrate a mechanism that is known generally in the art and is particularly similar to what is described in patent specification GB-A-1395319, both the tangential shift and the resulting pivotal movement are facilitated by mounting ball ends 37, 38 at opposite extremities of carriage 34, the two ball centres both lying on line 35. End 37 slides within a cylindrical socket 39 mounted on frame 30, while end 38 is captive within a ball-shaped socket formed in a piston 40, which slides within a cylinder 41 also mounted on frame 30. The chamber 42 of cylinder 41 is connected by way of control valve means 43 to the same pressurised fluid source 11 by which the end load chamber 10 is supplied. By using valve 43 to vary the fluid pressure within chamber 42, piston 40 imparts tangential shift to carriage 34. As already referred to, those movements will have the effect of causing the carriage and its roller 20 to tilt about line 35, and so to change the transmitted ratio.
The centre 22 of each roller 20 must at all times lie on the centre circle of the imaginary torus to whose surfaces races 6 and 14 conform, and when the ratio unit is in equilibrium--that is to say, when the transmitted ratio is constant for the time being--the spin axis 33 of each roller intersects the variator axis 2. In order that ratio-change should be brought about by a combination of components of tangential shift and rotation, as just described, a further geometrical feature is desirable and is illustrated in FIG. 3. This feature is that while the roller centre 22 lies in the central plane 50 of the imaginary torus at all times, ball end 37 lies to one side of that plane and ball end 38 to the other side, so that line 35 is inclined to plane 50 at an angle C known in the art as the castor angle. The effect of this angle may be explained as follows. If discs 4, 13 are rotating as indicated by arrows 51 and 52, the transmission of torque by rollers 20 between races 6 and 14 produces a torque reaction on each roller carriage 34, urging the associated piston 40 into its cylinder 41. For the transmission to be in equilibrium, two conditions must be fulfilled. Firstly the spin axis 33 of each roller must intersect the variator axis 2. Secondly the force exerted upon piston 40 by the fluid in cylinder 41 must be equal and opposite to the force which the torque reaction exerts upon the roller carriage, both forces being measured in a plane at right angles to the variator axis 2. If now the fluid pressure is increased in cylinder 41, driving the piston 40 downwards (as shown in FIG. 3) against the direction of the discs/roller torque reaction, equilibrium is destroyed because the cylinder and torque reaction forces are no longer in balance. The roller axis 33 will therefore no longer intersect the variator axis 2. As a result a steering force is imposed on the roller by the discs 4 and 13 so as to tilt the carriage 34 about line 35, until equilibrium is restored when the cylinder and torque reaction forces are in balance again, and when axis 33 once more intersects axis 2, the degree of tilt (which is proportional to the resulting change in transmitted ratio) being dependent upon the size of the initial tangential displacement or shift, and of the castor angle. Tangential shift in the opposite direction, which in the known variator shown in FIG. 3 will be brought about by a reduction in fluid pressure in cylinder 41, will result in the roller tilting in the opposite direction.
A fundamental feature of variators of the type just described in outline, and described in greater detail in patent specification GB-A-1395319 for example, is that they are of "force-balance" type. That is to say, one of the conditions that must be fulfilled for equilibrium of the transmission at any given ratio value is that the torque reaction force and the hydraulic force acting upon the carriage piston must be in balance. If either of these forces changes, equilibrium is lost until the forces are brought into balance once more. This feature distinguishes transmissions as shown in GB-A-1395319, and transmissions according to the present invention also, from an older generation of CVT's of the toroidal-race, rolling-traction type in which roller and carriage are positioned by mechanical means which are not themselves responsive to the torque reaction forces to which the rollers and carriages, once positioned, are themselves subjected. Patent specification U.S. Pat. No. 2,130,314 describes a mechanical positioning system of this kind, in which one end of the roller carriage is connected by a ball-and-socket joint to a control pinion. The transmitted ratio is varied by turning the pinion, so changing the orientation of the carriage by altering the location of its point of connection to the pinion. However, the carriage/pinion connection is such that the torque reaction experienced at the disc/roller interfaces through the carriage acts upon the pinion in a direction substantially parallel to its axis of rotation. No useful balance between the torque reaction force and the force applied to the pinion to turn it is therefore possible, and means other than force balance must therefore be found to ensure that the pinion always seeks the rotary position at which the roller transmits the ratio required by the instantaneous prevailing conditions.
In the known mechanism of force-balance type shown in FIGS. 2 and 3 the ball ends 37, 38 can move axially and simultaneously within their respective cylinders so that the line 35 moves, and each carriage 34 can rotate about the instantaneous position of line 35. However, because the carriage is located at both ends, it has no freedom to rotate about any other axis. FIG. 4 of the drawings of patent specification GB-A-1600972 (equivalent to U.S. Pat. No. 4,281,559) shows another variety of known mechanism in which the roller carriage (83) is fast with the head (82) of the single piston by which the position of the roller (13) is controlled. As with the two known mechanisms just described, this carriage is capable of translational movement along a line (the axis of movement of piston head 82) and of rotation about that line, but has no freedom to rotate about any other axis. It should also be noted that in the CVT shown in FIG. 4 of GB-A-1600972 the two rotors 10, 12 between which the roller 13 transmits traction must themselves be capable of simultaneous and equal movements, in a direction parallel to the main axis of the CVT, to accommodate displacements of the roller 13 by piston 82; the requirement for such movement of the rotors naturally introduces further complexity and expense for the CVT as a whole.
Another known design of CVT of the force-balance type is shown and described in patent specification U.S. Pat. No. 3,933,054, in which the traction forces experienced by each roller (40-42 in the drawings) are balanced by the hydraulic force acting on a piston (66). As to how to synchronise this balance of forces with the desired value of the transmitted ratio, the teaching of U.S. Pat. No. 3,933,054 is clear. Each roller carriage is connected by a hinged joint (pin 47) to the mechanism on which the piston (66) is mounted. The carriage also carries a cam follower (50) which engages with a cam slot (70) secured to the transmission casing. As already explained, in the description of FIGS. 1 to 3, two conditions must be fulfilled if a transmission of this type is to be in equilibrium. Therefore when the transmission of U.S. Pat. No. 3,933,054 falls out of equilibrium, two related but distinct motions must take place in order to restore It. Firstly there is a generally axial movement of each piston (66) within its cylinder (65), until a new torque reaction at the disc/roller interfaces balances a new hydraulic force exerted upon the piston by the fluid within the cylinder. Second, the angle of tilt of the roller (40-42) must change until the roller axis once more intersects the drive axis (D) of the transmission. Specification U.S. Pat. No. 3,933,054 teaches that the slot/follower (70/50) engagement is essential to achieve this second motion. In response to the first motion of the piston, the follower (50) is forced to move along the slot (70) so changing the angle of tilt of the roller (40-42) and thus the transmitted ratio. This requires both pivoting at the hinged joint (47), and rotation of the piston 66 about its axis within its cylinder (65). Now the axis of the hinged joint (47) intersects the two points of contact of the roller (41) with the disc grooves (30, 31), so without the engagement of follower (50) and slot (70) as taught by specification U.S. Pat. No. 3,933,054, the tilt angle of each roller (40-42) in response to any loss of equilibrium of the transmission, would be indeterminate. As taught by U.S. Pat. No. 3,933,054, therefore, for effective operation each assembly of carriage and roller thus requires four points of contact with adjacent mechanism, namely the contact between the roller and the two grooves between which it is transmitting traction, the contact with the hydraulic operating mechanism through the hinge (47), and the follower/slot contact.